System, apparatus, and method for direct chill casting venting

ABSTRACT

Provided herein is a system, apparatus, and method for venting a direct chill casting mold by venting excess casting gas and retaining oxide from atop a casting during the direct chill casting process. Methods of venting casting gas from a direct chill casting mold include: supplying the direct chill casting mold with molten metal through a transition plate; supplying a casting gas through a casting surface of the direct chill casting mold; venting the casting gas from a gas pocket in the transition plate, wherein venting the casting gas from the gas pocket in the transition plate is performed in response to a pressure of the casting gas in the gas pocket reaching a predetermined pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 63/073,523, filed on Sep. 2, 2020, the contents of which arehereby incorporated by reference in their entirety.

TECHNOLOGICAL FIELD

The present disclosure relates to a system, apparatus, and method forventing a direct chill casting mold, and more particularly, to ventingexcess casting gas and retaining oxide from atop a casting during thedirect chill casting process.

BACKGROUND

Metal products are formed in a variety of ways; however numerous formingmethods first require an ingot, billet, or other cast part that canserve as the raw material from which a metal end product can bemanufactured, such as through rolling, extrusion, or machining, forexample. One method of manufacturing an ingot or billet is through acontinuous casting process known as direct chill casting, whereby avertically oriented mold cavity is situated above a platform thattranslates vertically down into a casting pit. A starter block may besituated on the platform and form a bottom of the mold cavity, at leastinitially, to begin the casting process. Molten metal is poured into themold cavity whereupon the molten metal cools, typically using a coolingfluid. The platform with the starter block thereon descends into thecasting pit at a predefined speed to allow the metal exiting the moldcavity and descending with the starter block to solidify. The platformcontinues to be lowered as more molten metal enters the mold cavity, andsolid metal exits the mold cavity. This continuous casting processallows metal ingots and billets to be formed according to the profile ofthe mold cavity and having a length limited only by the casting pitdepth and the hydraulically actuated platform moving therein.

BRIEF SUMMARY

The present disclosure relates to a system, apparatus, and method forventing a direct chill gas cushion casting hot-top billet mold, and moreparticularly, to venting excess casting gas and retaining oxide fromatop a casting during the direct chill casting process. Embodimentsprovided herein include a transition plate for a direct chill castingmold including: a top surface, a bottom surface, where a casting gaspocket is defined at a periphery of the bottom surface, and one or morevent holes defined within the casting gas pocket. The transition plateof an example embodiment includes a lip that extends around theperiphery of the transition plate and is separated from the bottomsurface by a gas pocket surface. The one or more vent holes of anexample embodiment are defined in the gas pocket surface.

According to an example embodiment of the transition plate, the lip iselevated with respect to the bottom surface when the transition plate ispositioned on a mold, where the casting gas pocket is formed at theperiphery of the transition plate by the lip and the gas pocket surface,where the vent holes are disposed closer to the bottom surface than tothe lip. According to an example embodiment, in response to a gas bubbleforming in the casting gas pocket, the plurality of vent holes areconfigured to permit casting gas to be vented before the casting gasreaches the bottom surface of the transition plate. The gas pocketsurface of an example embodiment includes a chamfered surface relativeto the bottom surface, where the one or more vent holes are defined inthe chamfered surface. The plurality of vent holes of an exampleembodiment include a web of material that is gas permeable and notpermeable by molten metal. The plurality of vent holes of an exampleembodiment are vented to atmospheric pressure. The plurality of ventholes of an example embodiment are associated with a valve, where thevalve permits the plurality of vent holes to be vented to atmosphericpressure in response to pressure in the casting gas pocket satisfying apredetermined value. According to an example embodiment, the transitionplate includes a lip, where the casting gas pocket is defined betweenthe lip and the bottom surface.

Embodiments provided herein include a method of venting casting gas froma direct chill casting mold including: supplying the direct chillcasting mold with molten metal through a transition plate; supplying acasting gas through a casting surface of the direct chill casting mold;and venting the casting gas from a gas pocket in the transition plate,where venting the casting gas from the gas pocket in the transitionplate is performed in response to a pressure of the casting gas in thegas pocket reaching a predetermined pressure. The predetermined pressureof an example embodiment is determined based on a metallostatic headpressure of the molten metal supplied to the direct chill casting mold.The method of an example embodiment further includes: supplying pressureto a plurality of vent holes in the transition plate to prevent moltenmetal flow through the vent holes; and reducing or removing pressure tothe plurality of vent holes to allow venting of casting gas.

Embodiments provided herein include a system for venting a direct chillcasting mold including: a direct chill casting mold; a thimble throughwhich molten metal is supplied to the direct chill casting mold; atransition plate attached to the direct chill casting mold and intowhich the thimble is received, where the transition plate includes a gaschannel and a plurality of vents disposed therein, where in response tomolten metal filling the direct chill casting mold, casting gas isvented through the gas channel in the transition plate. The transitionplate of an example embodiment includes a top surface and a bottomsurface, where the casting gas pocket is defined at a periphery of thebottom surface.

According to a system of an example embodiment, the transition plateincludes a lip, where the lip extends around the periphery of thetransition plate and is separated from the bottom surface by a gaspocket surface. The one or more vent holes of an example embodiment aredefined in the gas pocket surface. The lip of a transition plate of anexample embodiment is elevated with respect to the bottom surface whenthe transition plate is positioned on a mold, where the casting gaspocket is formed at the periphery of the transition plate by the lip andthe gas pocket surface, and where the vent holes are disposed closer tothe bottom surface than to the lip. According to an example embodiment,in response to a gas bubble forming the int e casting gas pocket, theplurality of vent holes are configured to permit casting gas to bevented before the casting gas reaches the bottom surface of thetransition plate. The gas pocket surface of an example embodimentincludes a chamfered surface relative to the bottom surface, where theone or more vent holes are defined in the chamfered surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 illustrates an example embodiment of a direct chill casting moldaccording to the prior art;

FIG. 2 illustrates an example of the initial stages of direct chillcasting or continuous casting according to an example embodiment of thepresent disclosure;

FIG. 3 illustrates an example embodiment following the initial stages ofdirect chill casting according to an example embodiment of the presentdisclosure;

FIG. 4 illustrates an example embodiment of steady-state direct chillcasting according to an example embodiment of the present disclosure;

FIG. 5 illustrates air gap casting of a billet according to an exampleembodiment of the present disclosure;

FIG. 6 illustrates the casting gas pocket configuration in a transitionplate according to an example embodiment of the present disclosure;

FIG. 7 illustrates vent holes defined within a casting gas pocketaccording to an example embodiment of the present disclosure;

FIG. 8 is a flowchart of a method for venting casting gas from a directchill casting mold according to an example embodiment of the presentdisclosure; and

FIG. 9 illustrates a transition plate including an oxide dam accordingto an example embodiment of the present disclosure.

DETAILED DESCRIPTION

Example embodiments of the present disclosure now will be described morefully hereinafter with reference to the accompanying drawings, in whichsome, but not all embodiments of the disclosure are shown. Indeed,embodiments described herein take many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements. Like numbers refer to like elements throughout.

Embodiments of the present disclosure generally relate to a system,apparatus, and method for venting a direct chill casting mold, and moreparticularly, to venting excess casting gas and retaining oxide fromatop a casting during the direct chill casting process.

Vertical direct chill casting or continuous casting is a process used toproduce ingots or billets that have a variety of cross-sectional shapesand sizes for use in a variety of manufacturing applications. Theprocess of direct chill casting begins with a horizontal mold table ormold frame containing one or more vertically-oriented molds disposedtherein. Each of the molds defines a mold cavity, where the moldcavities are initially closed at the bottom with a starter block to sealthe bottom of the mold cavity. Molten metal is introduced to each moldcavity through a metal distribution system to fill the mold cavities. Asthe molten metal proximate the bottom of the mold, adjacent to thestarter block solidifies, the starter block is moved vertically downwardalong a linear path into a casting pit. The movement of the starterblock is caused by a hydraulically-lowered platform to which the starterblock is attached. The movement of the starter block vertically downwarddraws the solidified metal from the mold cavity while additional moltenmetal is introduced into the mold cavity. Once started, this processmoves at a relatively steady-state for a continuous casting process thatforms a metal ingot having a profile defined by the mold cavity, and aheight defined by the depth to which the platform and starter block aremoved.

During the casting process, the mold itself is cooled to encouragesolidification of the metal prior to the metal exiting the mold cavityas the starter block is advanced downwardly, and a cooling fluid isintroduced to the surface of the metal proximate the exit of the moldcavity as the metal is cast to draw heat from the cast metal ingot andto solidify the molten metal within the now-solidified shell of theingot. As the starter block is advanced downward, the cooling fluid issprayed directly on the ingot to cool the surface and to draw heat fromwithin the core of the ingot.

FIG. 1 depicts a general illustration of a cross-section of a directchill casting mold 100 during the continuous casting process. Theillustrated mold could be for a round billet or a substantiallyrectangular ingot, for example. The cooling water spray pattern asdescribed herein is primarily directed to round billet casting. However,embodiments could potentially be used for a substantially rectangularingot, particularly when the corners of said ingot have some degree ofcurvature. As shown, the continuous casting mold 105 forms a mold cavityfrom which the cast part 110 is formed. The casting process begins withthe starter block 115 sealing or substantially filling the bottom of themold cavity against mold walls of the continuous casting mold 105. Asthe platform 120 moves down along arrow 145 into a casting pit and thecast part begins to solidify at its edges within the mold walls of thecontinuous casting mold 105, the cast part 110 exits the mold cavity.Metal flows from a pouring trough 125, which in some embodimentsincludes a heated reservoir or a reservoir fed from a furnace, forexample, through thimble 130 into the mold cavity. As shown, the thimble130 is partially submerged within a molten pool of metal 135 to avoidthe oxidation of metal that would occur if fed from above the moltenmetal pool 135. The solidified metal 140 constitutes the formed castpart, such as an ingot. Flow through the thimble 130 is controlledwithin the pouring trough 125, such as by a tapered plug fitting withinan orifice connecting a cavity of the pouring trough 125 with a flowchannel through the thimble 130. Conventionally, the pouring trough 125,thimble 130, and mold cavity/mold walls of the continuous casting mold105 are held in a fixed relationship from the beginning of the castingoperation through the end of the casting operation. Flow of metalthrough the thimble 130 continues as the platform 120 continues todescend along arrow 145 into the casting pit. When the casting operationis to end, either by the platform being at the bottom of its travel, themetal supply running low, or the cast part reaching the completed size,the flow of metal through the thimble 130 stops, and the thimbleassembled on the trough is removed from the molten pool of metal 135 toallow the molten pool to solidify and complete the cast part.

FIG. 2 illustrates an example embodiment of a hot top casting method ofthe direct chill casting process according to the present disclosureincluding a continuous casting mold 105, trough 125, and thimble 130 forsupplying molten metal from the trough to the cavity of the mold. Theillustrated embodiment of FIG. 2 includes a starting position where thetip of the thimble 130 or thimble is positioned proximate the starterblock 115 which is supported by the platform 120. The starter block 115is positioned atop platform 120 and aligned to cooperate with the mold105 to seal the mold cavity and preclude molten metal 107 from leakingfrom between the continuous casting mold 105 and the starter block 115.The thimble 130 or thimble is received into a transition plate 200 thatis securely attached to the top of the mold 105, such as by threadedengagement. The transition plate 200 of an example embodiment is securedto the mold 105 by a metal ring that is threaded into a round openingatop the billet mold 105 to hold the transition plate securely to themold. The mold 105 of an example embodiment is constructed of a metalsuch as aluminum, while the thimble 130 and transition plate 200 aregenerally formed of a refractory material that is resilient to heat.

FIG. 2 illustrates the start of a cast with the starter block 115aligned with the continuous casting mold 105. As the cast starts shownin FIG. 3 , the platform 120 descends with the starter block 115 asmolten metal flows through the thimble 130 from the trough 125, andsolidifies on the starter block 115 and at the bottom of the mold cavityforming the cast part 140. In this manner, as the starter block 115descends away from the continuous casting mold 105, the cast part, shownin FIG. 4 as 140, is formed. FIG. 4 illustrates the run-state phase ofthe casting process or the steady-state portion where the platform 120descends at a near constant rate with the cast part 140 growingaccordingly. FIG. 2 also illustrates spray jets 150 that will bedescribed in greater detail below, where the spray jets provide acoolant or cooling fluid to the surface of the casting.

Direct chill casting using the hot top casting method of FIGS. 2-4 witha transition plate 200, while effective, has drawbacks. In particular,excess casting gas and oxides become trapped between the surface of themolten metal 107 and the transition plate 200.

According to example embodiments described herein, billet mold castingtechnology for hot-top direct chill casting of aluminum, as shown inFIG. 5 , employs a graphite casting surface 210 upon which the initialsolidification of the billet being cast occurs. The permeable graphitematerial allows for flowing both casting gas and casting lubricant tothe casting surface that produces an air-slip casting conditionincluding air gap 220 between the molten metal 107 that is solidifyingin the mold cavity and the graphite casting surface 210. The castinglubricant reduces the friction on the casting surface 210 to preventsticking and tearing of the freshly solidifying shell of the cast part140. The casting gas flow further aids in reducing this friction whileat the same time provides a thin film of gas between the casting surfaceand the billet shell which reduces the thermal heat transfer from themolten aluminum to the casting surface. When properly balanced, theintroduction of gas and oil produces an as-cast billet with a verysmooth surface and very narrow shell thickness as compared toconventionally cast billets. Water or coolant flowing to spray jets 150from the coolant chamber 155 impinges upon the shell of the cast part140 and proceeds to flow down the sides of the cast part as shown at 145to further cool the casting.

The amount of casting lubricant used during casting is directly relatedto the surface area of the billet. Balancing the amount of casting gasintroduced through the casting surface is difficult Due to the inherentshrinkage that occurs during the solidification process, the shell ofthe billet contracts away from the casting surface 210 slightly andallows the gas to escape out the lower portion of the mold cavity.However, the density of the casting gas is substantially lower than themolten metal, such that any excess casting gas that cannot escape outthe lower portion of the mold tends to rise upwards inside the moldcavity and up through the molten metal above the mold in the pouringtrough 125 or “hot top” design of the casting system. Further, an airtrapping recess or pocket of an example embodiment is fabricated intothe transition plate 200 or graphite casting ring forming the castingsurface 210 which captures the gas in a pocket 230 at the corner of themold cavity where the flowing liquid metal turns from a horizontaltrajectory to a vertical trajectory, and down along the casting surface.

FIG. 6 illustrates a section-view of a portion of a mold 105 includingthe transition plate 200 secured to the mold by a threaded collar 205.Also shown is the graphite casting surface 210 and the pocket 230 at thecorner that captures rising casting gas. When properly balanced, thecontinual flow of casting gas fills the pocket 230 and as the pressureincreases to the point that the pressure matches the metallostaticpressure of the metal in the trough 125 above, the gas flows downwardthrough the air gap 220 without bubbling up through the thimble 130.Bubbling up through the molten metal should be reduced or prevented inorder to prevent entrainment of oxide films into the metal above themold which are then pulled down into the solidifying billet. These oxidefilms are considered to be ‘inclusions’ which have the potential tocreate defects in subsequent downstream processed components.

The gas pocket 230 in a direct chill casting system described herein isthe area where the transition plate 200 meets the casting surface 210.This area is where the molten aluminum flows outward from the metal feedopening in the thimble 130 toward the mold wall and then changesdirection to flow downward to begin forming the solidifying shell. In ahot top casting configuration as shown in FIGS. 2-5 , the metallostaticpressure of the liquid metal head above the mold attempts to force themetal to completely fill this area and forms the pocket 230 of gas, andthe accumulated gas pressure combined with the alloy and strength of theoxide forms a critical radius commonly referred to as the ‘meniscus’radius. To aid in the formation of the meniscus radius and contain thetrapped gas, according to example embodiments described herein, a recessis fabricated into the transition plate at the casting surfaceinterface.

The gas pocket 230 of example embodiments is designed such that thewidth is kept close to the natural meniscus radius formed. The depth ofthe pocket 230 of an example embodiment is kept to a minimum in order toreduce the overall volume of the pocket. The edge of the pocket 230 ofan example embodiment is smoothed to reduce the tendency to tear theoxide layer as it moves along the hot metal face and transitions to thepocket and meniscus radius. During casting using the hot top method ofdirect chill casting described herein, a dynamic heaving or pulsingaction forms at the pocket 230 area. The gas bubble in the pocketincreases in size as does the pressure due to the continual influx ofcasting gas until the air bubble can force its way down between the moldwall and the casting along the air gap 220 and escape out of the bottomof the mold cavity. This increase in bubble volume forces metal back upthrough the thimble or thimble 130 such that when the gas pressure isreleased and gas escapes, the metal level lowers. A swaying or rockingharmonic may develop with the mold that is located directly across themetal delivery runner of the trough 125. This cyclical heaving of themeniscus should be reduced or kept to a minimum to prevent formation ofsurge laps, which are accompanied by a microstructural abnormality inthe solidifying billet shell that is generally shown as meniscus marks.These meniscus marks directly affect the total shell zone width, andthicker shell zones are undesirable for downstream processing when toopronounced.

A secondary reason to reduce or to keep the metal heaving to a minimumis that as the gas bubble in the pocket 230 increases in size, thebubble extends beyond the edge of the pocket 230 onto the hot metal faceadjacent to the transition plate 200. When excess casting gas releasesalong the air gap 220 and the bubble shrinks, the action splays theoxide layer across the edge of the pocket. As this occurs, the oxidelayer often tears which can lead to metal attachments to the pocket edgealong with random non-uniform oxide releases on the billet surface.

In a worst case scenario of example embodiments of hot top casting, thecasting gas flow rate is too great for the natural release of gas downand out through the bottom of the mold cavity, and the excess gas breaksout over the edge of the thimble 130 opening and releases bubbles upthrough the melt above the mold. This sudden escape of gas violentlycollapses the gas pocket and liquid metal completely fills into thearea. This event has several undesirable consequences leading to poorbillet surface quality. For example, a result includes a large heavyoxide release creating a non-uniform billet surface appearance. There isincreased potential for folding these heavy oxides that are subsurfaceinto the solidifying shell, and increased potential for attachments tothe transition plate pocket 230 area or the graphite casting surface 210as the protective oxide layer has been breached and liquid metalcontacts these surfaces. The collapse of the meniscus and exposure toliquid metal increase the potential for metal to penetrate into anysmall gaps at the transition plate to graphite casting ring interface orinto any type of excess gas venting scheme. Metal attachments can resultin scrapped billet and potential bleed-outs. The temperature of thecasting surface increases momentarily during the gas release from thepocket collapse that can lead to increased burning of the castinglubricant and potentially generate varnish, which is another potentialaluminum attachment point resulting in surface defects.

In addition to the above issues, casting gas bubbling up through thethimble 130 entrains oxide films in the melt as the oxygen in thecasting gas bubble is stripped and reacts with the molten aluminum toform these oxide films. The quality of the billet is diminished by theseoxides and surface issues resulting from casting gas movement. It isdesirable to eliminate casting gas bubbling up through the melt duringthe entire casting process to prevent the formation of inclusions.According to example embodiments described herein, embodiments reduce oreliminate casting gas bubbling up through the thimble 130 and throughthe molten metal to prevent oxide film entrainment. Eliminating anybubbling is a balancing act between allowing enough flow rate of castinggas applied to the mold to maintain an air gap 220 casting condition andrestricting the flow rate that escaping gas travels downward along theair gap interface and out through the lower portion of the mold ratherthan up through the molten metal delivery system. The correct amount ofcasting gas is directly related to the thermal conditions at the castingsurface. Colder casting conditions generally require higher casting gasflow rates than hotter casting conditions due to colder conditionscausing the solidification of the billet to occur higher on the castingsurface and much of the casting gas escapes out of the bottom of themold.

Hotter casting conditions move the solidification front further down thecasting surface allowing the casting gas to be more effective inmaintaining the air gap 220. These conditions also reduce the ability ofthe gas to be able to escape out of the bottom of the mold, therebybubbling up through the thimble 130. This situation creates a challengein that many casting operations pass through a significantly varyingmetal temperature range from the beginning of the cast through the endof the cast, thereby making it more difficult to optimize the castinggas flow rate to maintain the air gap 220 with minimal rocking of themelt and no bubbling through the thimble 130. However, even when melttemperatures are stabilized, the casting gas flow rate window remainsrelatively narrow to maintain the highest billet surface quality withoutlosing the air gap 220, generating surge laps, or bubbling. Losing theair gap 220 creates an inferior quality billet as compared to a billetwith surge laps, and may result in scrapping of the entire billet.Further, losing the air gap for any period of time may overheat thecasting surface and burn the casting oil, plugging the pores of thegraphite casting surface 210 thereby preventing gas flow, and requiringmold removal and replacement of the graphite casting ring.

Embodiments described herein include the ability to widen the window ofcasting gas flow rate without creating any bubbling issues as describedabove which increases the robustness of the casting. Venting of excesscasting gas as described herein enables operating with higher castinggas flow rates that ensures maintaining the air gap 220 at cold castingconditions while not allowing bubbling during hotter conditions.

According to an example embodiment described herein and illustrated inFIG. 7 , a cross section of a portion of a transition plate 200 isdepicted and described herein. The transition plate of the illustratedembodiment includes a top surface 238 and a bottom surface 248. Thetransition plate 200 further includes a rim 242 extending around acircumference of the transition plate, where the rim of the illustratedembodiment includes a lip 244. When the transition plate 200 is inposition in a casting mold 105, the lip 244 seals the top of the castingcavity against the mold. The lip 244 of the example embodiment is shownelevated relative to the bottom surface 248 of the transition plate 200.The elevated position of the lip 244 relative to the bottom surface 248of the transition plate 200 produces the casting gas pocket 230. The lip244 is joined to the bottom surface 248 of the transition plate by a gaspocket surface. The gas pocket surface (240) of the illustratedembodiment of FIG. 7 is a ramp or chamfer, though embodiments include afillet or radiused surface.

As shown, the transition plate 200 includes a vent hole 250 of aplurality of vent holes around the circumference of the transition platein the region of the pocket 230. The holes, an example embodiment ofwhich are 0.5 millimeters in diameter, are positioned along a ramp ofthe gas pocket surface 240 of the gas pocket 230 recess in thetransition plate. The vent holes 250 vent to a vent channel 260 to allowcasting gas to escape from the casting mold 105. When the gas pocketbubble grows due to the high gas flow rate, the edge of the bubble movesthe meniscus 245 down the ramped surface of the pocket along thedirection of arrow 255 preparing to breech the pocket edge and bubble upthrough the melt. However, when the leading edge of the enlarging bubblein the pocket 230 reaches the vent hole 250 on the ramp of the gaspocket surface 240, the gas pocket self-vents the excess gas. This typeof system includes orifices through which the gas escapes that are smallenough that the metal will not be able to penetrate the orifice due tothe surface tension of the molten metal.

In another example embodiment, a vent hole 250 and/or a vent channel 260are filled with a porous material that can be penetrated by gas, but notby molten metal. Such material includes a fibrous web of materialsimilar to a filter cartridge. The vent hole 250 of an exampleembodiment is filled with a porous material that provides a particulardegree of resistance to gas flow such that the vent hole is optionallypositioned in a variety of locations in the pocket 230, such that whenthe gas pressure in the pocket reaches a sufficient pressure, gas isleaked through the vent hole without requiring a particular position ofthe gas bubble to breech before venting.

While passive venting is employed as described in the embodiments above,active venting of the gap of an example embodiment provides analternative system that is configurable by a user. An example embodimentof such active ventilation includes a floating needle valve and seatarrangement that has been designed to crack open at a specific gaspressure in the transition plate 200 pocket 230. The pressure of anexample embodiment is selected to be a predetermined pressure, whichapproximately matches the metallostatic head pressure of the metal levelabove the mold. When the gas bubble in the pocket increases in size andresultant pressure, the needle lifts from its seat and the excesscasting gas escapes, thereby preventing gas from bubbling up through thethimble 130. Such a pressure relief valve 265 of an example embodimentis received within channel 260 of the transition plate 200 as shown inFIG. 7 . The pressure relief valve 265 of an example is calibrated to apredefined pressure, which is determined to be a pressure below whichcasting gas does not bubble up through the molten metal, and above whichcasting gas escapes in an undesirable path. Further, various pressurerelieving systems could be used for active ventilation of the gap toallow or prevent flow of gas from the gas pocket 230 during casting.While venting of the casting gas from the gas pocket 230 may be done toatmospheric pressure or ambient pressure of the casting environment,venting of gas from the gas pocket of an example embodiment is also beregulated by means of pressure control to either increase the amount ofgas vented by reducing pressure or increasing pressure to keep gas ventsclear as necessary

While venting of the gas pocket of the aforementioned embodiments isaccomplished through vent holes in the gas pocket as described above,embodiments optionally employ gas paths in the transition plate to guidegas as the gas is escaping from the gas pocket along a defined gas path.An embodiment includes sculpting paths in the transition plate 200 andother refractory components such as the thimble 130 to direct gas alonga path between the refractory pot shell and the liquid metal such that atrue bubble does not actually form that can float up through the thimble130 creating entrained oxides. Another example embodiment of crafting apath for the gas to escape is to create a chimney that allows the gas tobubble up towards and out of the metal flow into the mold. While oxidefilms may be generated in this embodiment, they would not becomeentrained in the cast billet. The concept of venting excess casting gasenables a much wider window for casting gas flow rates for ease ofmulti-strand operation (multiple billets concurrently) allowing forreduced meniscus pulsing and eliminating bubbling up through the melt.

FIG. 8 is a flowchart of a method for venting casting gas from a directchill casting mold. As shown, molten metal is supplied to a direct chillcasting mold through a transition plate as shown at 310. This moltenmetal of an example embodiment is provided through a trough (e.g.,trough 125) and a thimble (e.g. thimble 130). Casting gas is suppliedthrough a casting surface of the mold as shown at 320. The casting gasis supplied, for example, through casting surface 220 of a graphitecasting ring as shown in FIGS. 2-6 . Venting of the casting gas isperformed from the gas pocket in the transition plate as shown at 330.The transition plate includes a gas pocket that receives the casting gasand as pressure builds, the casting gas is vented through the mechanismsdescribed above.

Blocks of the flowchart support combinations of means for performing thespecified functions and combinations of operations for performing thespecified functions for performing the specified functions. It will alsobe understood that one or more blocks of the flowcharts, andcombinations of blocks in the flowcharts, can be implemented by variousaspects of venting of casting gas from a direct chill casting mold asdescribed above.

In some embodiments, certain ones of the operations above are modifiedor further amplified. Furthermore, in some embodiments, additionaloptional operations are included. Modifications, additions, oramplifications to the operations above of an example embodiment areperformed in any order and in any combination that facilitates theventing of casting gas as described herein.

In another embodiment a valving system is used to pressurize the ventingholes during the metal filling stage of the cast. Metal spilling intothe mold can become turbulent which can force liquid metal into thesmall venting holes or porous media, effectively plugging off theability to vent the excess casting gas. Applying a positive gas flowthru the venting system helps to mitigate this problem of metalpenetration. The valving system switches from positive flow into themold cavity, to free flow venting of the gas pocket once the mold isfilled with metal and the starting block begins to descend into thecasting pit. This valving system could be a separately controlled andoperated process, or can be incorporated into the existing casting gassupply porting in the mold itself and use varying casting gas pressureto shuttle between applying positive flow to venting the excess gas.This is not only useful to help prevent metal penetration during moldfilling, but also to prevent the vents from being plugged when thecasting operators are applying a release agent coating to the hot metalface of the transition plate 200 between casts.

Transition Plate Oxide Dam

Additional embodiments of a transition plate include a transition plate‘oxide dam’ where, in the case of hot top billet casting, the term‘oxide dam’ refers to an undercut recess in the transition plate fromthe thimble 130 or thimble area toward the mold bore. The use of anoxide dam creates a condition where the majority of oxide on the head ofthe billet is trapped and unable to break off and roll over into ontothe as-cast billet surface. The hot metal face is greatly reduced and assuch, the oxide layer is much thinner and easily maintains mobilityflowing outward and rolling over the meniscus and onto the as-castbillet surface. This result leaves the surface of the billet veryuniform in appearance and prevents random heavy oxide releases or‘patches’ from breaking free during the cast and disturbing theappearance of the billet. A narrow hot metal face also helps toeliminate the need to ‘hit’ the mold hard with a high gas flow rate tobreak free the heavy oxides that form from the cascading metal duringmold filling.

FIG. 9 illustrates two transition plates 200, with the transition plateon the right being conventional and including a pocket 230 around theperiphery where the transition plate engages the mold cavity. Thetransition plate 200 on the left includes the pocket 230 around theperiphery, but also includes an undercut 270, not present in surface 280of the conventional transition plate. The undercut provides an area inwhich the thimble 130 will sit below the bottom surface of the undercutproviding an oxide dam as the oxides atop the molten metal will beretained within the undercut, while clean, molten metal will flowbeneath the undercut, past the pocket 230 and transition down the sideof the casting.

Applicant has found that an optimum undercut within the transition plateof an example embodiment of around twelve millimeters deep in order toreliably retain the oxide as the metal head heaves gently up and downwith the meniscus pulling lightly due to the air gap casting condition.The hot metal face is generally kept to around twelve to twentymillimeters. This distance is a compromise both to help prevent the gasbubble that forms at the meniscus from breaching over the edge of theoxide dam and bubbling up through the thimble opening, and to limit thetime the oxide has to ‘grow’ in thickness and strength before rollingover the meniscus.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A transition plate for engaging a direct chillcasting mold comprising: a top surface; a bottom surface, wherein thebottom surface defines an upper surface of a mold cavity of the directchill casting mold, wherein a casting gas pocket is defined at aperiphery of the bottom surface by a gas pocket surface where thetransition plate meets a wall of the direct chill casting mold; and oneor more vent holes defined within the gas pocket surface configured tovent the casting gas pocket.
 2. The transition plate of claim 1, furthercomprising a lip, wherein the lip extends around the periphery of thetransition plate and is separated from the bottom surface by the gaspocket surface.
 3. The transition plate of claim 2, wherein the lip iselevated with respect to the bottom surface when the transition plate isengaged with the direct chill casting mold, wherein the lip forms anupper surface of the gas pocket, and wherein the one or more vent holesare defined through the gas pocket surface closer to the bottom surfacethan to the lip.
 4. The transition plate of claim 3, wherein in responseto a gas bubble forming in the casting gas pocket, the one or more ventholes are configured to permit casting gas to be vented before thecasting gas reaches the bottom surface of the transition plate.
 5. Thetransition plate of claim 1, wherein the gas pocket surface comprises achamfered surface relative to the bottom surface, wherein the one ormore vent holes are defined in the chamfered surface.
 6. The transitionplate of claim 1, wherein the plurality of vent holes are comprise a webof material that is gas permeable and not permeable by molten metal. 7.The transition plate of claim 1, wherein the one or more vent holes arevented to atmospheric pressure.
 8. The transition plate of claim 1,wherein the one or more vent holes are associated with a valve, whereinthe valve permits the one or more vent holes to be vented to atmosphericpressure in response to pressure in the casting gas pocket satisfying apredetermined value.
 9. The transition plate of claim 1, furthercomprising a lip, wherein the casting gas pocket is defined between thelip and the bottom surface by the gas pocket surface.
 10. A method ofventing casting gas from a direct chill casting mold comprising:supplying the direct chill casting mold with molten metal through atransition plate; supplying a casting gas through a casting surface ofthe direct chill casting mold; and venting the casting gas from a gaspocket in the transition plate, wherein the casting gas is ventedthrough one or more vent holes defined in a gas pocket surface disposedat a periphery of the transition plate where the transition plate meetsa wall of the direct chill casting mold, the gas pocket surface definingthe casting gas pocket wherein venting the casting gas from the gaspocket in the transition plate is performed in response to a pressure ofthe casting gas in the gas pocket reaching a predetermined pressure. 11.The method of claim 10, wherein the predetermined pressure is determinedbased on a metallostatic head pressure of the molten metal supplied tothe direct chill casting mold.
 12. The method of claim 10, furthercomprising: supplying pressure to the one or more vent holes in thetransition plate to prevent molten metal flow through the one or morevent holes; and reducing or removing pressure to the one or more ventholes to allow venting of casting gas.
 13. A system for venting a directchill casting mold comprising: a direct chill casting mold; a thimblethrough which molten metal is supplied to the direct chill casting mold;and a transition plate attached to the direct chill casting mold andinto which the thimble is received, wherein the transition platecomprises a gas pocket surface and one or more vents disposed therein,wherein in response to molten metal filling the direct chill castingmold, casting gas is configured to vented through the one or more ventsin the transition plate.
 14. The system of claim 13, wherein thetransition plate comprises: a top surface; a bottom surface and whereinthe gas pocket surface extends about a periphery of the transitionplate, wherein a casting gas pocket is defined at a periphery of thebottom surface by the gas pocket surface.
 15. The system of claim 14,wherein the transition plate further comprising a lip, wherein the lipextends around the periphery of the transition plate and is separatedfrom the bottom surface by the gas pocket surface.
 16. The system ofclaim 15, wherein the one or more vent holes are defined in the gaspocket surface.
 17. The system of claim 16, wherein the lip is elevatedwith respect to the bottom surface when the transition plate ispositioned on a mold, wherein the casting gas pocket is formed at theperiphery of the transition plate by the lip and the gas pocket surface,and wherein the one or more vent holes are disposed closer to the bottomsurface than to the lip.
 18. The system of claim 17, wherein in responseto a casting gas bubble forming in the casting gas pocket, the one ormore vent holes are configured to permit the casting gas bubble to bevented before the casting gas bubble reaches the bottom surface of thetransition plate.
 19. The system of claim 15, wherein the gas pocketsurface comprises a chamfered surface relative to the bottom surface,wherein the one or more vent holes are defined in the chamfered surface.